Electron microscope



April 15, 1941.

I H. BoERscH ELECTRON MICROSCOPE Filed March 26, 1940 F i .1. 3 2 I I 5 1 U Fi .5. Fig.2.

Inventor-r Hans Boersch,

His Abtorneg.

Patented Apr. 15, 1941 ELECTRON mcnoscorn Hans Boersch, Berlin, Germany, assignor to General Electric Company, a corporation of New York Application March 26, 1940, Serial No. 326,105

In Germany February 20, 1939 v Claims. (01, 250-495 The present invention relates to improvements in electron microscopes.

In the art of electronic miscroscopy it is known that magnification effects may be obtained by projecting a divergent beamof electrons from a point source through an object to be investigated. While a very great degree of manification may be obtained in this way if a proper relationship exists between the distance'from the point source to the object and the distance from the said source to the image reproducing surface, considerable difliculty has been experienced in providing an electron point source of conveniently usable character. It is an object of my present invention to provide such a source.

In accordance with the invention it is proposed to utilize the combination of means for producing a concentrated parallel-ray beam of electrons and means, in the form of a minute solid body appropriately arranged in the beam path, for producing sharp divergence of the beam-rays as the beam approaches the object under investigation.

The features which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the drawing, in which Fig. 1 is a diagrammatic crosssectional view of an electron microscope suitably embodying the invention; Fig. 2 illustrates. a detail of Fig. 1, and Fig. 3 is an enlarged fragmentary view showing an alternative modification of one part of the apparatus of Fig. 1.

The electron microscope illustrated in Fig. 1 includes a cathode whose effective emitting surface approximates a point, e. g. a hairpin filament I. The cathode cooperates with an appropriately charged accelerating cylinder! which in this case serves simultaneously as an enclosure for the microscope as a whole. trons projected from the cathode are controlled and concentrated into a parallel-ray beam by means of a Wehnelt cylinder 3. In the path of the beam there is provided an object support l-that is to say, a screen on which may be mounted an object 5 which is to be investigated.

A magnified image of the object 5 is observed on a. fluorescent screen or other image-reproducing surface provided at 6 on the end wall of the enclosure.

1 The beam emitted from the cathode l is accelerated by the anode and directed toward the object 5. In front of the object are one or more The elecdiaphragms l which assure that all elements of the beam which proceed toward theobject pursue substantially parallel paths. In order to avoid undesirable-influencing of the beam, the diaphragm 1 maybe constructed in the form of a cone having its apex directed toward the oathode, or else there maybe employed two or more diaphragms with progressively larger orifices as shown in the drawing. At a point very close to the objecti the electron beamis caused to impinge'on a minute solid body which causes a sharp divergence of the electron rays. For example, the body may, as shown in Fig. 2, consist of a solid object 9 of essentially point dimensions supported on a knife-edge support as indicated at In. For example, one may use in this connection a minute crystal of a metal or a salt. Under certain circumstances no separate scattering body need be provided, but one may employ instead one of the minute irregularities which occur on the edge of a sharpened metal object such as a razor blade or a diaphragm having a thin edged aperture. In any of these arrangements it will be understood that the divergent cone of electrons produced by the scattering effect of the scattering body will project a greatly enlarged silhouette of the object 5 on the screen 6. As previously indicated, the amount of magnification thereby obtained will be a function of the ratio of the distance between the scattering body and the object and the distance between the scattering body and the image-reproducing surface. A magnifying arrangement of the character described will yield good magnification and resolution at relatively low voltages of operation.

It may be pointed out that the occurrence of magnification of the type under consideration depends only upon the development of a beam having some usable part thereof in which the rays are sharply divergent, as from a point, and that it is not necessary to assume the existence of a beam which diverges symmetrically in all its parts. For example, in connection with a construction such as that of Fig. 1 the portion of the beam which passes nearest to the deflecting particle 9 will be characterized by sharply divergent rays, these being due to the extreme non-uniformity of the electric field in proximity to the particle. At distances relatively remote from the particle equivalent divergence may not occur, but this is immaterial since it is not necessary to use the entire cross-section of the beam in the magnifying experiment.

Care must be taken that only that portion of the beam which is affected by the scattering device actually falls upon the object. The other portion of the beam may be expediently intercepted by the supporting structure associated with the scattering device. Thus, if a razor blade is used as explained above, the blade may be pushed into the electron beam for a distance such that it screens ofi the entire beam with the exception of a small part which is permitted to be scattered by some irregularity existing at the blade edge.

Fig. 3 shows a case in which special provision is made for screening the unscattered portion of the electron beam. In this case the illustrated arrangement includes a support l0 having a sector-shaped openingin which is placed a scattering device 9'. By this arrangement the unscattered portion of the beam is interceptedby the screening efi'ect of the support III.

For the scattering body one may in some cases employ an insulator, for example, a bit of mica or of Celluloid.

In general, the scattering results as a consequence of the repulsion of the beam by the negative charge acquired by the scattering body. In order to be able to regulate magnification obtained with the apparatus described above it is expedient to make either the fluorescent screen or the object support or the scattering device movable in an axial direction.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An electron microscope comprising means for supporting an object to be investigated, an image-reproducing surface materially spaced from said object support, means more remote from said image-reproducing surface than said object support for projecting a parallel-ray beam of electrons toward said object support, and a discrete electron-impervious solid body of essentially point dimensions positioned between the beam-projecting means and the object support at a location extremely close to the latter element, the said solid body being arranged in the path of the said electron beam so as to produce a divergent scattering of the rays of the beam which impinge on the body.

2. Electron microscope according to claim 1, in which the said solid body consists of a minute crystal.

3. Electron microscope according to claim 1, in which the said solid body consists of a minute irregularity upon the sharpened edge of a metallic object.

4. Electron microscope according to claim 1, in which the said solid body consists of an insulating material, whereby charging of the body assists in producing divergence of the electron rays.

5. Electron microscope according to claim 1, in which means are provided for screening off the electron rays which do not impinge on the said solid body.

HANS BOERSCH. 

